64 SYNTHESIS VIII 



In the investigation of carotenoid epoxides and their isomeric furanoid 

 oxides, the spectral properties of these compounds are of much importance. 

 The following are some empirically recognised regularities: 



Conversion of a carotenoid pigment into a mono-epoxide results in a 

 displacement in the absorption bands towards the violet. The displacement of 

 the longest wavelength band amounts, on the average, to 8 m// in carbon 

 disulphide. The formation of a di-epoxide results in a displacement of the bands 

 by about 17 m/n. A rather larger displacement (about 21 mpi) of the absorption 

 bands towards shorter wavelength accompanies the change of a mono-epoxide 

 into the isomeric furanoid oxide. For a di-epoxide, the difference is about 

 twice as great. These regularities allow a fairly certain prediction of the ab- 

 soiption spectra of carotenoid epoxides and their furanoid isomers. 



Of the carotenoid epoxides and furanoid oxides so far prepared, the following 

 have up to now been found in nature : 



a-Carotene mono-epoxide 



Flavochrome 



(/5-Carotene mono-epoxide)* 



Citroxanthin = Mutatochrome 



Zeaxanthin mono-epoxide = Antheraxanthin 



Zeaxanthin di-epoxide = Violaxanthin 



Xanthophyll mono-epoxide 



Flavoxanthin 



Chrysanthemaxanthin 



Auroxanthin 



Rubichrome 



Trollixanthin", a pigment recently isolated from the blossoms of trollius 

 europaeus, also possesses an epoxide structure. 



The wide distribution of carotenoid epoxides in plants raises the question 

 as to their physiological significance. No definite answer can be given, but the 

 ready loss of oxygen suggests the possibility that these epoxides play a part 

 in biological oxidation processes in the plant. Further investigations are required 

 to elucidate this problem. 



REFERENCES 



1. Cf. R. KuHN and co-workers, Angew. Chem. 50 (1937) 7°3; ^^^- ^9 (^93^) I757. 19791 

 71 (1938) 1889. 



2. P. Karrer, a. Helfenstein and R. Widmer, Helv. Chim. Acta 11 (1928) 1201. 



3. P. Karrer and co-workers, Helv. Chim. Acta 15 (1932) 1218, 1399. 



* /3-Carotene mono-epoxide has not yet been found in nature, but as mutatochrome (^ 

 citroxanthin) is a natural pigment and almost certainly formed from /3-carotene mono- 

 epoxide, it is very probable that the latter also occurs in plants. 



